Method for electrolytically treating the surface of a steel plate with a chromate solution

ABSTRACT

WITH RESPECT TO THE PRIOR ART METHOD WHICH COMPRISES IMMERSING A STEEL PLATE AS A CATHODE IN AN ELECTROLYTE IN WHICH THERE ARE DISSOLVED CHROMIC ACID OR SALTS THEREOF CONTAINING CHROMIUM VI OXIDE AS A MAIN AGENT AND A THIOCYANIC COMPOUND AS A PROMOTING AGENT, AND SUBJECTING THE SURFACE OF SAID STEEL PLATE TO ELECTROLYTIC TREATMENT SO AS TO IMPROVE ITS CORROSION RESISTANCE AND THE ADHESIVITY OF PAINT THERETO, IT HAS BEEN FOUND THAT ADDITION OF ALUMINUM SODIUM FLUORIDE OR ALUMINIUM POTASSIUM FLUORIDE TO SAID ELECTROLYTE AS A SECOND PROMOT-   ING AGENT WILL ELEVATE THE ADHESIVITY OF PAINT TO THE TREATED SURFACE OF THE STEEL PLATE.

a 19M HIDEHISA YAMAGISHI ETAL 3,735,940

METHOD FOR ELECTROLYTICALLY TREATING THE SURFACE OF A STEEL PLATE WITH A CHROMATE SOLUTION 2 Sheets-Sheet 1 Filed Jan. 8, 1973 FBG.

CONCENTRATION OF MF '3N0F (9/2) FEGZ pm42O8642O 3 5 w xo 222310 52%;: 6 mmbj m5 2 CONCENTRATION OF AQF -3N0F (g/Q) Jam. 1%, H|DEH|5A YAMAGISH] ETAL 3,785,940

METHOD FOR ELECTROLYTICALLY TREATING THE SURFACE OF A STEEL PLATE WITH A CHROMATE SQLUTION 2 Sheets-Sheet 2 Filed Jan. 8, 1973 FHGIS CONCENTRATION OF MP 3NGF (g/R) United States Patent Office 7/6,313 Int. c1. 02% 11/02,- czsr 17/00 US. Cl. 204-56 R 3 Claims ABSTRACT OF THE DISCLOSURE With respect to the prior art method which comprises immersing a steel plate as a cathode in an electrolyte in which there are dissolved chromic acid or salts thereof containing chromium VI oxide as a main agent and a thiocyanic compound as a promoting agent, and subjecting the surface of said steel plate to electrolytic treatment so as to improve its corrosion resistance and the adhesivity of paint thereto, it has been found that addition of aluminium sodium fluoride or aluminium potassium fluoride to said electrolyte as a second promoting agent will elevate the adhesivity of paint to the treated surface of the steel plate.

This invention relates to a method for electrolytically treating the surface of a steel plate with a solution of chromate so as to coat the surface of steel plate with a duplex layer having an excellent property of rendering the steel surface very resistant to corrosion and elevating the adhesivity of paint thereto.

Heretofore, there have been proposed various methods for subjecting the surface of a steel plate, particularly a thin steel plate to the so-called electrolytic chromate treatment to form a duplex layer of metallic chromium and hydrated chromium oxide on the surface of said steel material, thereby elevating the corrosion resistance and visual appeal of said surface and the adhesivity of paint thereto. All the proposed methods use an electrolyte containing chromic acid or salts thereof dissolved at a concentration of about g. to about 100 g. as CrO per liter, namely, at a lower concentration than is used in a metallic chromium plating alone, and a promoter, and consists in depositing by electrolytic reduction a thin duplex layer in one step on the surface of steel material by immersing it as a cathode in the electrolyte for a few minutes. Referring to said promoter, Japanese Pat. No. 269,930 proposes aromatic sulfonic acid or salts thereof, Japanese Pat. No. 524,872 (the corresponding US. Pat. No. 3,484,347) thiocyanic compounds, and Japanese patent publication No. 2,768/68 sulfuric acid or salts thereof.

A steel plate whose surface is electrolytically treated with a solution of chromate by the known methods has an excellent appearance and a strong corrosion resistance, but is still handicapped by the slightly low adhesivity of paint to said surface. Where a steel plate is subjected to very deep drawing after its surface is coated with paint, part of the coated paint unavoidably comes off.

We have discovered that the low adhesivity of paint results from the unduly large thickness of a layer of hydrated chromium oxide forming the upper portion of the duplex layer deposited on the surface of a steel plate, and that proper reduction of said thickness will noticeably improve the adhesivity of paint to the surface of a steel plate without affecting its corrosion resistance.

For control of the thickness of a layer of the abovementioned hydrated chromium oxide, the present inven- 3,785,940 Patented Jan. 15, 1974 tors previously invented a method (U.S. Pat. No. 3,679,- 554) which improved the process of using thiocyanic compounds as a promoter for the aforesaid surface treatment by further adding a proper amount of hydrosilicofluoric or borofluoric compounds as a second promoter for free control of the thickness of the layer of hydrated chromium oxide, thereby enabling the deep drawing of a steel plate whose surface is coated with paint without causing the paint to come off.

However, said method has been found to have the under-mentioned drawbacks. Namely, the hydrosilicofluoric or borofluoric compounds added as a second pro moter gradually corroded lead or alloys thereof used as an anode, resulting in the appreciable depletion of the anode. As the result, the anode fell off little by little to form a sludge in an electrolyte, causing an electrolytically deposited layer to be marred by said sludge.

This invention has been accomplished for further improvement of the foregoing method (US. Pat. No. 3,679,554). The present invention is characterized in that a second promoter consisting of aluminium sodium fluoride AlF '3NaF or aluminium potassium fluoride AIF '3KF is substituted for that of the aforesaid hydrosilicofluoric or borofluoric compounds, thereby offering the advantage of permitting the growth of a thinner, more uniform and compact layer of hydrated chromium oxide than has been possible with immediately preceding invention, substantially preventing the depletion of the anode, attaining the higher electrolytic efficiency of forming a base layer of metallic chromium than has been realized by any of the former inventions and reducing the time of electrolysis with the resultant elevation of productivity.

Other important objects and advantageous features of this invention will be apparent from the following description and accompanying drawings, wherein, for the present purpose of illustration only, the specific embodiments of this invention are set forth in detail.

In the drawings:

FIG. 1 is a curve diagram showing the relationship of the thickness of a base layer of metallic chromium (said thickness is expressed in the amount of metallic chromium per unit surface area of steel material) included in a duplex layer deposited on said steel surface and the concentrations of the first and second promoters, as combined between the case Where the method of this invention was used and the other case;

FIG. 2 is a curve diagram showing the relationship of the thickness of a layer of hydrated chromium oxide constituting the upper portion of the duplex layer and the concentration of the second promoter, with that of the first promoter kept unchanged; and

FIG. 3 is a curve diagram showing the relationship of the results of the screw cap test for the peel off of paint coated on the surface of steel plate as compared between the case where the method of this invention was applied and the other case.

There was proposed a method (Japanese patent publication No. 10,640/ 69) of treating the surface of iron and steel product which comprised adding only the aforesaid aluminium potassium fluoride as a promoter to an aqueous solution of chromic acid. However, the method had the drawbacks that absence of compounds having a thiocyanic group in an electrolyte caused a layer of metallic chromium constituting the base portion of a duplex layer deposited on the surface of iron and steel material to have a very small thickness, but instead, a layer of hydrated chromium oxide forming the upper portion of said duplex layer to have an unduly large thickness, and that, though the surface treated product displayed an elevated corrosion resistance, yet the paint coated on the surface of said product indicated too low adhesivity to effect the deep drawing of the product under a satisfactorily coated condition.

An electrolyte according to the method of this invention should preferably contain to 150 g./l. of chromium VI oxide. Where the concentration of the chromium VI oxide exceeds said range, a layer of hydrated chromium oxide is not ubstantially formed. Where said concentration grows smaller than said range, said layer becomes exclusively thick, causing the surface of steel material to be undesirably tinted yellow or orange.

The proportion of a thiocyanic compound contained in an electrolyte according to this invention is chosen to account for to of the main agent of chromium oxide. Formation of a layer of metallic chromium on the surface of steel material presents considerable difliculties where the electrolyte only consists of chromium oxide whose concentration falls within the aforesaid range of 10 to 150 g./l. Deposition of a layer of metallic chromium is made possible only when the electrolyte contains the aforesaid thiocyanic compound.

Where there is used an electrolyte prepared by adding only a thiocyanic compound to an aqueou solution of chromate, the electrolytic etficiency of depositing a layer of metallic chromium varies with the ratio of the con centration of the thiocyanic or SCN group to that of the chromium oxide CrO However, a maximum electrolyzing efliciency is about percent on an average under the electrolyzing conditions where the temperature is set at 50 C. and the current density at the cathode is a./dm. Said efficiency little varies from that of ordinary electrolytic plating of chromium metal. Where the electrolyte contains only a thiocyanic compound as a promoter, then a layer of hydrated chromium oxide deposited on the surface of steel material is very compact and strongly resistant to corrosion. However, said layer of hydrated chromium oxide is unduly thick to be tinted blue, preventing paint-coated steel material from being subjected to deep drawing under a satisfactory condition, namely, without the peel off ofthe paint.

For resolution of the above-mentioned problem, the immediately preceding invention (U.S. Pat. No. 3,679,554) was proposed which was based on the discovery that addition of hydrosilicofluoric or borofluoric compounds as a second promoter decreased the thickness of a layer of hydrated chromium oxide and moreover control of the addition of said second promoter enabled free adjustment of the thickness of said layer. However, said invention was later found to be still accompanied with the drawback that, as previously described, the second agent of said hydrosilicofluoric or borofluoric compound gradually corroded the anode, which crumbled little by little into the electrolyte to form a sludge therein, eventually obstructing electrolysis.

Accordingly, the present inventors searched for a new second promoter substituting the above-mentioned compounds which would not corrode the anode. As the result, it has been discovered that aluminium sodium fluoride (AlF -3NaF) known as the so-called artificial cryolite is adapted to meet the above-mentioned requirement. It has also been found that aluminium potassium fluoride (AlF -3KF) displays the same effect as the aluminium sodium fluoride. The concentration of both double salts should preferably account for 3 to of that of the chromium VI oxide used as a main agent. If each of said double salts is added to the electrolyte at higher concentration than then not only said addition will fail to indicate an increased effect, but also there will be presented ditficulties in the dissolution of the double salt in the electrolyte.

Though the electrolytic efliciency of forming a layer of metallic chromium according to the method of this invention may slightly vary with the concentration of thiocyanic compound and aluminium alkali fluoride in the electrolyte, yet the present invention displays a prominently high electrolytic efficiency under an optimum condition. FIG. 1 is a curve diagram showing the relationship of the concentrations of thiocyanic compounds and aluminium sodium fluoride in an electrolyte and the deposited amount of metallic chromium, Where electrolysis was carried out under the following conditions:

Content of CrO in the electrolyte g./l Current density of the cathode a./dm. 25 Temperature of the electrolyte /C 50 Time of electrolysi sec 3.5

The curve A represents the case where the electrolyte did not contain thiocyanic compounds. The curves B, C and D denote the cases where the electrolyte contained 0.3, 0.6 and 0.9 g./l. of sodium thiocyanate respectively. As seen from FIG. 1, where the concentration of the thiocyanic compound is fixed at any of the above three values, then the aluminium sodium fluoride can be added to the electrolyte at such optimum concentration as allows metallic chromium to be deposited at a maximum rate. The optimum concentration of said aluminium sodium fluoride varies with the concentration at which said thiocyanic compound is added to the electrolyte. FIG. 1 also shows that in the absence of thiocyanic compounds, the deposition of metallic chromium would be extremely small, regardless of the addition of the aluminium sodium fluoride.

Referring to FIG. 2, the curve B shows variation in the content of chromium in a deposited layer of hydrated chromium oxide, where electrolysis was carried out 3.5 seconds at a current density of 25 a./dm. and a temperature of 50 C., using various kinds of electrolyte prepared by adding diflFerent amounts of aluminium sodium fluoride to a solution in which there were dissolved 100' g./l. of CrO and 0.3 g./l. of sodium thiocyanate. The curve F presents in comparison with the curve B the data obtained where electrolysis was carried out under the same conditions, excepting that the electrolyte did not contain sodium thiocyanate. As apparent from the curve B of FIG. 2, a layer of hydrated chromium oxide formed by the method of this invention had a thickness of less than 0.2 mg./dm. as expressed in chromium, namely, far smaller than the thickness (0.2 to 0.4 mg./dm. of a layer of hydrated chromium oxide electrolytically deposited by the method of the immediately preceding invention (U.S. Pat. No. 3,679,554), using an electrolyte containing hydro silicofluoric or borofluoric compounds as the second promoter. Therefore, the present invention enables a paintcoated steel plate to be deep drawn with the peel off of the paint more assuredly prevented by that extent. Further, a steel plate treated by the method of the present invention presents a substantially transparent surface. In contrast Where surface treatment was effected using an electrolyte containing only aluminium sodium fluoride as a promoter but not a first promoter as in the case of the curve F, then the resultant layer of hydrated chromium oxide had a thickness of 1.3 to 1.6 mg./dm. as expressed in chromium, giving rise to the drawbacks that the paint coated on the surface of a steel plate came off When the plate was deeply drawn, and that the treated steel surface turned yellow.

FIG. 3 is a curve diagram showing the plotted results of the screw caps tests conducted on the samples of coated steel materials whose surfaces were subjected to chromate treatment using an electrolyte according to the present invention containing thiocyanic compounds as a first promoter and aluminium sodium fluoride as a second promoter, and the samples of coated steel materials whose surfaces were similarly subjected to chromate treatment using an electrolyte only containing aluminium sodium fluoride as a promoter. The curves G and H of FIG. 3 represent the cases where the electrolyte consisted of 100 g./1. of CrO 0.3 g./l. of NaSCN and 0 to 5 g./l. of AlF -3NaF, and the curves I and J the cases where the electrolyte consisted of 100 g./l. of CrO and 0 to 5 g./l. of AlF -3NaF. In all the cases, electrolysis was carried out 3.5 seconds at a cathode current density of 25 a./

dm. and a temperature of 50 C. Electrolytically treated samples of steel material were coated with a phenol-epoxy paint to a thickness of 50 mg./dm. The paint was baked minutes at a temperature of 205 C. Thereafter the samples were put to the screw cap test, the results of which are presented in FIG. 3. The curves G and I denote the samples whose outer surface was coated with the aforesaid paint, while the curves H and I represent the samples whose inner surface was coated with the same paint. The screw cap test was carried out by determining the extent of the peel off of the paint in the screwed portion of the screw cap, and the results were obtained on the 10-mark basis in comparison with those indicated by referential samples. Namely, 10 marks were given to the case where the tested sample did not indicate the peel off of the paint, and a zero mark was allowed to the case where more than 50 percent of the paint came off. FIG. 3 clearly shows that a steel plate whose surface was electrically treated with an electrolyte prepared by the method of this invention attained the prominent adhesivity of paint to the surface of said steel plate.

A comparison of electrolytic efficiency is given in Table 1 below between the case where the electrolyte contained aluminium sodium fluoride as a second promoter according to the present invention and the case where the electrolyte contained borofluoric acid as a second promoter according to the immediately preceding invention (US. Pat. No. 3,679,554). In both cases, electrolysis was carried out 3.5 seconds at a cathode current density of 25 a./dm. and a temperature of 5 0 C.

TABLE L-COMPARISON OF ELECTROLYTIC A comparison of the extent of anode corrosion is given in Table 2 below between the aforementioned cases.

TABLE 2.-COMPARISON OF ANODE CORROSION Weight loss of anode by cor rosion Composition of (mg./ electrolyte (g./l.) dmfi) Condition of corrosion Depletion due to corrosion indeed took place, but the surface did not come off noticeably.

The surface came ofi prominently, giving rise to irregularities over the entire steel surface.

Though diflicult of dissolution in, demineralized water, aluminium sodium fluoride and aluminium potassium fluoride are easily dissolved in an aqueous solution of chromic acid, if it is heated to higher temperature. The method of this invention can be applied, as is customarily practiced, at an electrolyzing temperature of 30 to 70 C. and a cathode current density of 10 to 100 a./dm. The more elevated the electrolyzing temperature, the more decreased the deposition of metallic chromium and hydrated chromium oxide, and the more increased the cathode current density, the higher the electrolytic efficiency of forming a layer of metallic chromium.

The present invention will be more fully understood by reference to the examples and referential experiments which follow.

6 EXAMPLE 1 A cold-rolled steel strip used for manufacturing a tinplate was electrolytically degreased 5 seconds at a temperature of C. and current density of 10 a./dm. using an electrolyte consisting of an aqueous solution of 30 g./l. of ortho-sodium silicate. After washed with water, the steel strip was electrolytically pickled 5 seconds at room temperature and a current density of 3 a./dm. using an electrolyte containing 10 g./l. of sulfuric acid. Thereafter the steel strip was subjected to electrolytic chromate treatment with an electrolyte consisting of 50 g./l. of CrO 0.5 g./l. of NaSCN and 3 g./l. of

with the anode prepared from an alloy of 93% Pb-7% Sn. Electrolysis was carried out 3.5 seconds at a temperature of 50 C. and a cathode current density of 25 a./dm. Said chromate treatment deposited. on the surface of the steel strip a colorless transparent duplex layer, the upper portion of which consisted of a layer of hydrated chromium oxide having a thickness of 0.22 mg./dm. as expressed in metallic chromium contained therein and the base portion of which was formed of a layer of metallic chromium having a thickness of 1.40 mgJdmF. The aforementioned screw cap test conducted on this steel strip gave a mark of 8 for the inner surface and a mark of 10 for the outer surface.

EXAMPLE 2 .The steel strip was subjected to the same pretreatments as in Example 1. Thereafter, the steel strip was subjected to an electrolytic chromate treatment with an electrolyte consisting of g./l. of CrO 0.3 g./l. of NaSCN and 5 g./l. of AIF -SKF, with the anode prepared from an alloy of 99% Pia-1% Ag. Electrolysis was carried out under the same conditions as in Example 1. The chromate treatment formed on the surface of the steel strip a colorless transparent duplex layer, the upper portion of which consisted of a layer of hydrated chromium oxide having a thickness of 0.12 mg./dm. as expressed in metallic chromium contained therein, and the base portion of which was formed of a layer of metallic chromium having a thickness of 1.74 mg./dm. The screw cap test conducted on the coated steel strip gave a mark of 10 for both outer and inner surfaces of the steel strip.

EXAMPLE 3 The same pretreatments as in Example 1 were applied to the steel strip, which was later subjected to the electrolytic chromate treatment with an electrolyte consisting of g./l. of CrO 0.5 g./l. of NaSCN and 3.0 g./l. of AlF -3NaF. Electrolysis was carried out under the same conditions with the anode prepared from the same material as in Example 1. Said chromate treatment deposited on the surface of the steel strip a duplex layer, the upper portion of which consisted of a colorless transparent layer of hydrated chromium oxide having a thickness of 0.08 mg./dm. as expressed in metallic chromium contained therein, and the base portion of which was formed of a layer of metallic chromium having a thickness of 1.52 mg./dm. The screw cap test conducted on the coated steel strip gave a mark of 10 for both outer and inner surfaces of the steel strip.

Referential experiments There were made eight referential experiments where electrolysis was carried out 3.5 seconds at a temperature of 50 C. and a cathode current density of 25 a./dm. but using an electrolyte whose components were added at concentrations departing from the respective ranges specified by the method of the present invention. The reof a layer of hydrated chromium oxide colored the sur face of said sample.

TABLE 3.REFERENTIAL EXPERIMENTS Content of Deposited chromium Value of screw amount of in hydrated cap test Composition of electrolyte (g.ll.) metallic chromium chromium oxide layer Color of steel plate Inner Outer N 0. C103 NaSCN NaaAlFs (mg/(1m?) (mgJdmJ) surface surface surface 150 0.5 0.5 0.5 0.30 Brown 7 9 150 16.0 5.0 0.32 0. 02 Colorless and transparent- 10 10 100 5. 0. 56 1. 44 Yellow 0 0 100 1. O 0 1. 0.35 Blue to brown 6 8 5 0. 05 1. 0 0. 18 2. 64 Yellow O 0 250 2. 5 2. 0 0.98 0. 01 Colorless and transparent- 10 100 0. 6 i 2. 0 1. 40 0. 29 Slightly brown.-- 7 9 100 1 1. O 0 1.00 0. 32 Brown 6 8 In Table 3, No. 1 had a thin layer of metallic chromium due to AlF -3NaF being added to the electrolyte at an unduly low concentration, with the resultant thick formation of a layer of hydrated chromium oxide, causing the surface of the steel plate to be tinted.

No. 2 was too thinly coated with a layer of metallic chromium because the electrolyte contained an excessively large amount of NaSCN. Accordingly, a layer of hydrated chromium oxide substantially disappeared.

N0. 3 was thinly coated with a layer of metallic chromium due to the absence of NaSCN, and in consequence excessively coated with a layer of hydrated chromium oxide, resulting in the coloration of the surface of said sample, and the failure of its deep drawing under a satisfactory coated condition.

No. 4 had its surface tinted, because a layer of hydrated chromium oxide became too thick due to the absence of AIF;, 3NaF in the electrolyte.

No. 5 was coated with an unduly thick layer of hydrated chromium oxide because of the too small content of CrO in the electrolyte, causing the failure of the deep drawing of said sample under a satisfactory coated condition.

No. 6 was treated with an electrolyte containing too much CrO and in consequence was plated with chromium to an ordinary extent of chromium plating.

No. 7 was treated with an electrolyte containing HBF as a second promoter according to the immediately preceding invention (US, Pat. No. 3,679,554), but was coated with a layer of hydrated chromium oxide more thickly than when the method of this invention was applied, leading to the slight coloration of the surface of said sample.

No. 8 was treated with an electrolyte which did not contain a second promoter, but sulfuric acid in substitution for NaSCN as a first promoter. Excess deposition What we claim is:

1. In a method for electrolytically treating the surface of a steel plate wherein the steel plate is immersed as a cathode in a solution containing 10 to grams per liter of chromium VI oxide and the solution is electrolyzed to deposit a thin duplex layer of metallic chromium and hydrated chromium oxide on the surface of said steel plate, an improvement being that the electrolyte consists of a chromic acid compound as the main agent, thiocyanic compounds as the first promoter and a second promoter selected from the group consisting of aluminium sodium fluoride and aluminium potassium fluoride.

2. The method according to claim 1 wherein the thiocyanic compound is present in the electrolyte at a concentration equal to to by weight of the chromium VI oxide contained in the electrolyte.

3. The method according to claim 1 wherein the compound of aluminium alkali fluoride is present in the electrolyte at a concentration equal to to ,6 by weight of the chromium VI oxide contained in the electrolyte.

References Cited UNITED STATES PATENTS 3,679,554 7/1972 Yamagishi et al. 204-56 R FOREIGN PATENTS 1,031,792 6/ 1966 Great Britain 20456 R JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204-38 E 

